Constant speed drives (CSD's) are typically used to convert variable speed motive power from a prime mover into constant speed motive power for driving a generator so that it produces constant frequency electrical power. Known CSD's are of the hydromechanical type which have been found to encounter difficulties in certain applications. For example, when such CSD's are used in aircraft, it has been found that the drives are sensitive to aircraft attitude changes and that maintenance intervals are shorter than desired.
Electrically-compensated CSD's have been proposed for use in applications where conventional CSD's have been found to be inadequate. Known electrically-compensated CSD's include a mechanical differential speed summer having a first input shaft coupled to the output shaft of the prime mover, a second input shaft and an output shaft at which the constant speed motive power is developed. A speed-compensation link is coupled to the second input shaft of the speed summer and includes a first or compensating permanent magnet machine having a power shaft coupled to the second input of the differential speed summer and electrical power windings. A motive power shaft of a second or control permanent magnet machine is coupled to either the output shaft of the prime mover or the output of the differential speed summer. Electrical power is transferred between the windings of the first permanent magnet machine and electrical power windings of the second permanent magnet machine by either a DC link inverter or a cycloconverter so that the compensating permanent magnet machine develops compensating speed of a magnitude and direction which causes the output of the differential to be driven at the desired constant speed.
The DC link inverter for controlling the flow of power between the windings of the permanent magnet machines has been found to cause excessive current flow in one of the machines at certain operating speeds. The cycloconverter was thought to be capable of superior performance as compared to the DC link inverter; however, this type of power converter is also subject to numerous generating difficulties. Chief among these is that the cycloconverter presents a low power factor load to the permanent magnet machines, in turn requiring unduly large and heavy machines. Also, at low speeds the cycloconverter can cause generation of pulsating torque at frequencies which are sufficiently low to permit response thereto by the mechanical components in the system. Further, at low operating speed the permanent magnet machines do not produce enough voltage to permit natural commutation of SCR's in the cycloconverter.
There is a need, therefore, for a power converter which can be used in an electrically-compensated constant speed drive that is not subject to the difficulties noted above.